II.
RADIO ASTRONOMY
Academic and Research Staff
R. A. Batchelor
D. C. Papa
G. D. Papadopoulos
Prof. D. H. Staelin
J. W. Barrett
Prof. A. H. Barrett
Prof. B. F. Burke
Prof. R. M. Price
Graduate Students
S.
F.
L.
A.
M.
H.
P.
C.
A.
POLARIZATION
D. L. Thacker
J. W. Waters
A. R. Whitney
W. J. Wilson
P. C. Myers
P. W. Rosenkranz
J. E. Rudzki, Jr.
P. R. Schwartz
Ewing
Hinteregger
Kebabian
Knight
MEASUREMENTS ON THE CLOUD 2 OH SOURCE
From the
OH emission was detected from interstellar dust clouds in 1967 (Heiles ).
measurement
of the ratio of the 1667-MHz to the 1665-MHz line strengths,
it was
believed that this emission was due to "normal" thermal emission (Heiles2).
If this
Since many
OH emission were due to thermal emission, it should also be unpolarized.
of the OH emission sources are nonthermal and have significant amounts of circular
polarization, we decided to measure the circular polarization of one normal OH emission source to see if it was, in fact, unpolarized.
Observations of the normal OH emission were made during March 1969, using the
Harvard College Observatory's 84-ft (26-m)Agassiz Radio Telescope. Because the dustcloud OH emission is
an extended source,
and the polarization properties
believed to be extended, the size of the antenna is unimportant.
are
also
The data were taken in
30-min intervals, alternating between right and left circular polarization in successive
intervals.
A load-switching mode, switching against a sky horn, was used.
The first
stage of the receiver was an uncooled parametric amplifier which gave a system noise
temperature of 150 0 K.
1.5-kHz filter receiver.
The spectral-line
processing
was
done
using
a
50-channel
The total integration time was 10 h with right circular and 10 h
with left circular polarization.
Data processing was carried out with the SLReduce program at the Haystack Lincoln
Laboratory Facility (Ball 3 ).
With this program, the data were averaged together, base-
line corrections made, and the sum So(RC+LC) spectrum and the difference S3(RC-LC)
spectrum plotted.
The circular polarization of the normal OH source Cloud 2 (1950:
a = 4
38
30
,
6 = 250 38') was measured because this is the strongest of the normal OH sources
This work was supported by the National Aeronautics and Space Administration
(Grant NGL 22-009-016), the National Science Foundation (Grants GP-14854, GP-13056,
and GP-14589), the Joint Services Electronics Progams (U.S. Army, U.S. Navy, and
U.S. Air Force) under Contract DA 28-043-AMC-02536(E), and U.S. Navy (Office of
Naval Research) under Contract N00014-67-A-0204-0033.
QPR No. 98
(II.
RADIO ASTRONOMY)
(Cudaback
and Heiles4).
Heiles
2
has
summarized
the
Cloud 2 parameters.
The
results of the Agassiz measurements are shown in Fig. 1I-1 where the So and S 3
1667-MHz spectra are plotted.
2.5
0O
0
J
-0.5
ii
0.25
1.25 >
0
-0.25
2
4
6
RADIAL
Fig. II-1.
8
10
VELOCITY (kM/s)
The 1667-MHz OH emission spectrum of the interstellar
dust cloud, Cloud 2.
The unpolarized, So(RC+LC), and
the difference,
S3(RC-LC),
spectra are plotted.
Integra-
tion times were 10 with RC, and 10 with LC polarization. Observations were made using the Harvard College
Observatory's 84-ft Agassiz Radio Telescope.
The 1667-MHz emission from Cloud 2 has less than 8 ± 2% circular polarization at
-1
the peak velocity of 5. 5 km sec
, and less than 15 ± 2% circular polarization over the
3-dB width of the line.
Therefore, this measurement supports the conclusion that the
1667-MHz emission is thermal.
It was also found that the Zeeman splitting is
QPR No. 98
less than 1.1 kHz, which makes the
RADIO ASTRONOMY)
(II.
magnetic field in Cloud 2 less than 10
-3
G.
It should be noted that this is not a very good
upper limit, since magnetic fields in interstellar dust clouds are probably 1-2 orders
of magnitude less than this value (Verschuur 5 ).
A recent measurement of the 1720-MHz emission from dust clouds shows that the
1720-MHz signal is much stronger than expected for thermal emission (Turner ).
Thus
it appears that the dust cloud OH emission is not normal in every respect.
We would like to thank Dr. J.
A.
Ball and the staff of the Agassiz Station of Harvard
College Observatory for the use of their facilities and technical support. We would also
like to thank the staff of the Haystack Microwave Research Facility, Lincoln Laboratory,
M. I. T. , for support in the data reduction.
W. J.
Wilson, A. H. Barrett
References
1.
C.
Heiles, Astrophys. J.
151,
919 (1968).
2.
C.
Heiles, Astrophys. J.
157,
123 (1969).
3.
J. A. Ball, Technical Note 1968-22, Lincoln Laboratory, M. I. T. , Lexington, Massachusetts.
4.
D. D. Cudaback and C. Heiles, Astrophys. J.
5.
G.
6.
B. E.
B.
VERY LONG BASELINE INTERFEROMETRY
L. Verschuur, Phys. Rev. Letters
Turner, Private communication,
155,
L21 (1969).
21, 775 (1968).
1970.
A successful Very Long Baseline Interferometer experiment was concluded between
the Haystack Microwave Research Facility, of Lincoln Laboratory, M. I. T. , the National
Radio Astronomy Observatory,
and the Maryland Point telescope of the Naval Research
Laboratory, at a frequency of 22. 235 GHz, the water-line frequency.
masers associated with W49,
The interstellar
W3 OH, the Orion Nebula, and VY Canis Majoris were
all observed successfully, and upper limits were placed upon the angular size of a
number of the observed features.
in Table II-1.
It is
stellar dimensions;
The results of the experiment are
shown that (i) the typical linear sizes of H 20 masers are
(ii)
and (iii) there are no fundamental bar-
riers to building phase-stable equipment for these frequencies.
2Tr
rad/min,
of
the atmosphere is sufficiently stable to permit VLBI obser-
vations at considerably shorter wavelengths;
stability,
summarized
is
attributable
entirely
to the frequency
The observed phase
stability
of the
Rubidium standards that were used in the experiment.
We are now preparing a three-station experiment utilizing the Haystack telescope,
the
140-ft Greenbank telescope,
QPR No. 98
and the 36-ft Kitt Peak telescope of the National
Table II- 1.
Source
VR
(km/s)
F ringe
Spacing
Experimental results.
Fringe
Amplitude
(sec of arc)
Angular
Size
(sec of arc)
Distance
Linear
Feature
Size
Size of
OH
Features
(kparsec)
(A. U.)
(sec of are)
0. 012
0. 3
< 0. 01
2.4
<24
. 005-. 02
3.5
.005
0.6
<
.003
0.5
<
1.5
<10
9
.005
0.6
<
.003
<
1.5
VY CMa
18
.006
0. 9
<
.004
<
2. 0
W49 (OH-l)
-8
.005
0. 7
<
.003
-6
.006
0.9
<
.003
<40
-2
.004
0.7
<
.003
<40
5
.005
0.9
<
.003
<40
7
.005
0.7
<
.003
<40
W3 (OH)T
Orion A
-49
Rest Frequency = 22235. 08 MHz.
tNRAO-NRL Baseline.
0. 5
15
<40
<10
-.
05
(11.
Radio Astronomy Observatory.
RADIO ASTRONOMY)
Three observing sessions will be carried out at these
stations during the month of June 1970.
B.
C.
F.
Burke,
D.
C. Papa, G.
Papadopoulos,
P.
R.
Schwartz
PULSAR FACILITY
During the past quarter, work continued on improving the performance of the receiver
to be used on the Pulsar Facility.
This has included testing the use of a broadband RF
preamplifier in conjunction with various filters to accept the desired band of frequencies
(approximately 1 MHz centered on 148 MHz) and provide adequate image rejection.
In May 1970, maintenance was performed on the array of 16 antennas to repair damage caused by the effects of the winter weather.
This included replacing some feed-
element damage when high winds knocked over several feed tripods.
been rephased and its impedance checked.
The array has since
The receiver will be tested on the array early
in the summer.
R. IM. Price, B. F. Burke
D.
PULSAR OBSERVATIONS WITH A SWEPT-FREQUENCY
LOCAL OSCILLATOR
Observations of pulsars have continued using the swept-frequency local-oscillator
(SLO) techniques described previously (Sutton et al. ).
near 120,
Observations have been made
160, and 230 MHz on a number (approximately 15) of pulsars available to the
300-ft telescope of the National Radio Astronomy Observatory.
We have noted in MP0031 and AP2015, two pulsars with well-known "marching" characteristics, that there are times when the apparent P 3 or rate of marching varies by
as much as a factor of two. This effect is particularly noticeable in MP0031, as is shown
in Fig. II-2. Note that the changes sometimes occur while the pulses are strong, and
PULSAR" OFF
PULSE NUMBER
(I OFF" TIME NOT TO SCALE)
O
MP0031
2
PULSE NUMBER
AP2015
Fig. 11-2.
QPR No. 98
P3 as a function of time for MP0031 and AP2015.
(II.
RADIO ASTRONOMY)
at other times while the pulses are weak or "off".
It has been suggested (Huguenin and
2
Taylor ) that this change occurs in a way related to the P4 phenomena reported by
3
Our present data are not extensive enough to support or contradict that
Staelin et al.
Studies of this phenomenon will continue.
suggestion.
1.5ms
0.
ms
AP2015
CP1919
PSR2218
Fig. 11-3.
6
HP1642
Average pulse shapes (5 min) at 160 MHz for 4 pulsars.
Amplitudes are not to the same scale.
Average pulse shapes of a number of pulsars are being accumulated. Figure 11-3
shows the 5-min average of pulsars CP1919, AP2015, HP1642, and PSR2218. Note that
1919 and 2015 have similar shapes as do 1642 and 2218. A study is under way to correlate the average pulse shapes with other pulsar characteristics.
R. M. Price, D. H. Staelin, J. M. Sutton
(Dr. John M. Sutton is at the National
West Virginia.)
QPR No. 98
Radio Astronomy
Observatory,
Greenbank,
RADIO ASTRONOMY
(II.
References
M. Price, and R. Weimer, Astrophys. J.
1.
J. M. Sutton, D. H. Staelin, R.
(1970).
2.
G. R.
Huguenin and J.
3.
D. H.
Staelin, M. S. Ewing, and R.
QPR No. 98
H. Taylor, Private communication,
M.
159,
1970.
Price, Astrophys. J.
160,
L7 (1970).
L89